Carrier detection circuits are typically used in modems and other devices to detect the presence of frequency shifted and/or phase shifted carrier signals. To understand such carrier detection circuits, it is helpful to examine the operation of a signaling protocol which is often employed in modems. One such protocol often used in modems is the IEEE 802.4 carrier band modem protocol which is illustrated in FIG. 1 in the form of a graph showing time on the horizontal axis and signal amplitude on the vertical axis. In this protocol, information is conveyed via a frequency shift keyed (FSK) signal having two different frequency components. That is, the signal information of this protocol is conveyed by a combination of 5 MHz signals designated L (half cycle of 5 MHz) and 10 MHz signals designated H (full cycle of 10 MHz). A bit of information is conveyed or encoded on a carrier during each of a plurality of successive medium access cycle (MAC's), designated MAC.sub.1, MAC.sub.2, MAC.sub.3, MAC.sub.4, . . . . Each MAC is defined to exhibit a time period equal to the period of a 5 MHz signal, that is, one full cycle thereof.
To convey a logical 1 during an MAC, a full cycle of 5 MHz (or LL) is transmitted during that MAC. Thus it may be said that a full cycle of 5 MHz (two successive half cycles of 5 MHZ) is the symbol which represents a logical 1. To convey a logical 0 during another MAC, two successive full cycles of 10 MHz (or HH) are transmitted during that MAC. Thus, as seen in FIG. 1, to simplify matters, LL=1, HH=0, and LH and HL represents non-data. In more detail, in the example of FIG. 1, an LL representing a logical 1 is transmitted during MAC.sub.1 and an HH representing a logical 0 is transmitted during MAC.sub.2. During MAC.sub.3 and MAC.sub.4 an HL and an LH are respectively transmitted, each representing non-data.
In the past, the carrier detect circuits employed with such protocols as the IEEE 802.4 carrier band modem protocol have been relatively simple threshold envelope detectors which detect when the received signal has exceeded a predetermined threshold level. When the received signal does exceed a predetermined threshold level, a window is opened and the signal is said to be present for a fixed period of time. Unfortunately, noise in such systems can randomly open this window as well as the desired received signal can open the window. To reduce the incidence of false carrier detection, it is desirable that only received signals exhibiting a signal level greater than a first level and less than a second level be detected. Otherwise, the carrier detect signal and/or modem coupled thereto may not function properly. Those skilled in the art recognize that it is desirable to reduce the number of false carrier detections as much as possible.
As discussed earlier, the IEE 802.4 carrier band modem protocol employs two frequencies of transmission, namely 5 and 10 MHz, to convey data. When these two frequencies are transmitted over cable, these two frequencies are attenuated by different amounts, respectively. This differential attenuation effect becomes greater as the length of the cable is increased. Conventional carrier detection circuits will generally detect a valid signal at the beginning of a transmission if either one of these two frequencies exceeds the predetermined threshold level for carrier detection.